Piero Guerrini

An active acoustic tripwire for simultaneous detection and localization of multiple underwater intruders

An algorithm allowing simultaneous detection and localization of multiple submerged targets crossing an acoustic tripwire based on forward scattering is described and then evaluated based upon data collected at sea. This paper quantifies the agreement between the theoretical performance and the results obtained from processing data gathered at sea for crossings at several depths and ranges. Targets crossing the acoustic field produce shadows on each side of the barrier, for specific sensors and for specific acoustic paths. In post-processing, a model is invoked to associate expected paths with the observed shadows. This process allows triangulation of the targets position inside the acoustic field. Precise localization is achieved by taking advantage of the multipath propagation structure of the received signal, together with the diversity of the source and receiver locations. Environmental robustness is demonstrated using simulations and can be explained by the use of an array of sources spatially distributed through the water column.

SLITA: A new slim towed array for AUV applications

The typical 70 mm diameter towed array was developed for blue‐water detection at long range and low frequencies in the 1960s. Since then, there has been a need for towed arrays that are lighter and less expensive, especially since the maturing field of autonomous vehicles has expanded the potential of such arrays. The marriage of AUVs and lightweight towed arrays is a natural progression in the development of littoral autonomous sensing networks for applications such Anti‐Submarine Warfare, marine mammals, or ambient noise measurements. In August 2007, NURC began to design and build a new thin diameter (31 mm) high‐frequency (up to 20 kHz) nested towed array for ASW purposes. An engineering at‐sea trial of the array towed by OEX AUV was performed beginning of November 2007. The flow noise level of the array while towed and the potential influence of the AUV self‐noise on the acoustic array were also measured. This paper will first describe the array design, its acquisition system and its integration on th...

Experimental demonstration of a high-frequency forward scattering acoustic barrier in a dynamic coastal environment

Detecting a target by measuring its forward scattered field is of interest for harbor surveillance because target strength levels are generally higher in the forward direction than in the backward direction for simple geometries. An acoustic barrier based on forward scattering was demonstrated in a nearly range-independent shallow water environment. The experimental location was characterized by high reverberation, low temporal signal coherence, and, as a result, few stable multipath arrivals due to the fluctuating sea surface. This high-frequency experiment utilized a vertical source array, broadcasting broadside pulses, and a vertical receiver array spanning the water column. The signal of interest was the aberration (in space and time) caused by the acoustic forward scattering field of crossing targets (2-m-long aluminum cylinder, 1-m-diameter steel sphere and pair of scuba tanks). Hence, the spatial and temporal coherence of the recorded acoustic signals was first investigated to assess the stability of the early acoustic arrivals in this rapidly fluctuating coastal environment. A principal component analysis of the stable portion of the recorded acoustic signals was then used to determine the crossing time of the target and to isolate some of its scattered wavefield components.

Tank measurements of scattering from a resin-filled fiberglass spherical shell with internal flaws.

This paper presents results of acoustic inversion and structural health monitoring achieved by means of low to midfrequency elastic scattering analysis of simple, curved objects, insonified in a water tank. Acoustic elastic scattering measurements were conducted between 15 and 100 kHz on a 60-mm-radius fiberglass spherical shell, filled with a low-shear-speed epoxy resin. Preliminary measurements were conducted also on the void shell before filling, and on a solid sphere of the same material as the filler. These data were used to estimate the constituent material parameters via acoustic inversion. The objects were measured in the backscatter direction, suspended at midwater, and insonified by a broadband directional transducer. From the inspection of the response of the solid-filled shell it was possible to detect and characterize significant inhomogeneities of the interior (air pockets), the presence of which were later confirmed by x-ray CT scan and ultrasound measurements. Elastic wave analysis and a model-data comparison study support the physical interpretation of the measurements.

Localization of small surface vessels through acoustic data fusion of two tetrahedral arrays of hydrophones

Detection and tracking of vessels is important in confined areas such as marine parks or harbors. Nowadays, the presence of ships can be accurately monitored either by radar or via AIS system, while small vessels, which have weak radar signature, may be easily missed. The paper presents the detection and localization algorithms optimized for small- and mid-sized boats and based on data either from a single underwater sensor station of four hydrophones, or from data fusion between two hydrophone volumetric arrays. Each platform hosts a sparse tetrahedral array of broadband hydrophones and pan, tilt, compass and depth sensors. Both acoustic and non-acoustic data from the two stations are transferred to shore, where they are stored and processed on a PC. The basis of localization algorithm is the cross-correlation between pairs of hydrophones along time (crosscorrelogram). The wavevector estimation of a vessel from each tetrahedron is achieved through Least Mean Square method. Adequate data association algor...

A High‐Frequency Active Underwater Acoustic Barrier Experiment Using a Time Reversal Mirror; Model‐Data Comparison

An underwater acoustic barrier based upon forward scattering in a Time‐Reversal Mirror (TRM) was experimentally demonstrated for the first time in 2000 by Song et al.. The barrier consisted of a TRM, a vertical receive array (VRA) and a co‐located probe source working at 3,500 Hz in an ocean waveguide near the western coast of Italy. In April 2003 further barrier tests were performed by applying for the first time at sea a new method that provided the capability to focus at different depths by using only the TRM transducers without the complication of additional probe sources. An echo repeater towed by R/V Alliance crossed the barrier, emulating the field forward scattered by a possible intruder insonified by the TRM. The presence of a target between the time reversal mirror and the focus can be detected if it significantly disturbs the quiescent region. A normal mode code is used to model the sound propagation in a waveguide. This is applied to predict the unperturbed and perturbed focused acoustic field...

Hydroflown: MEMS‐based Underwater Acoustical Particle Velocity Sensor

The increasing problems related to homeland security and harbour/infrastructure protection have increased the level of interest on vector sensors. Market surveys carried out during the last three years gave the conclusion that there is a need for a new generation, small size, and low‐cost underwater sensors capable of measuring particle velocity in three dimensional plain within a broad frequency band (2 Hz ‐ 50 kHz) and with high angular resolution. The small size MEMS‐based sensors developed by Microflown Technologies BV Inc are the worlds only commercially available transducers that are capable of measuring the particle velocity, instead of pressure, in air. The development of a new generation, innovative and low‐cost underwater sensors and technologies based on that in‐air nano technology is therefore considered. This technology has a great potential to become a revolutionary underwater acoustic sensor using nano‐technology, capable of finding many applications like sensors for Autonomous Underwater ...

Surveillance and Protection of Underwater Archaeological Sites: Sea‐Guard

Recent advances in underwater detection and survey techniques have increased the possibility of finding the wrecks of vessels and their cargoes. Since prehistoric times, many populations have used vessels for transportation and trade and built harbors and it is now estimated that there are about a million antique wrecks that lie underwater, still to be discovered. Many of these wrecks and ancient harbours lie within shallow depths and can be discovered and accessed easily by humans using available hardware such as side‐scan sonars, multi‐beam echo sounders, ROVs, GPS navigation, diving gear. Even though the majority of known archaeological sites on land are well protected by guards and high tech equipment, there is no technology for surveillance and protection of underwater archeology sites. This paper presents a new system specifically adapted for the surveillance and protection of underwater archaeological sites (SEA‐GUARD). Options available to the acoustic sensor suite of this system are pressure hydr...

Performance of a high‐frequency acoustic forward‐scatter barrier in a dynamic coastal environment

This paper investigates the performance of a high‐frequency ‘‘barrier’’ or ‘‘tripwire’’ surveillance system based on forward scattering for shallow water (<20 m depth) choke points, such as harbor entrance. Harbor entrances are usually regions of very high ambient noise and often are reverberation‐limited environments, making them challenging for some traditional ASW techniques, i.e., techniques inherited from blue‐water ASW. Detecting a target by measuring its forward‐scattered field is of interest because sound scattered in the forward direction is generally higher than in the backward direction. We will present a series of proof‐of‐concept experiments to test the feasibility of an acoustic tripwire based on forward scattering using two vertical source and receive arrays operated over a 135‐m range at a center frequency of 15 kHz with about 8 kHz of bandwidth. The signal of interest was the aberration (in space and time) caused by the acoustic forward scattering field of a crossing object (scuba tank, 2...

Tank measurements and modeling of elastic scattering by resin‐filled fiberglass spherical shells

Acoustic elastic scattering measurements were conducted in a tank on 6 cm‐radius fiberglass spherical shells filled partially or completely with a low‐shear‐speed epoxy resin. Preliminary measurements were conducted also on the void shell before filling and on a solid sphere of the same material of the filler, in order to estimate the constituent material parameters via acoustic inversion. The objects were measured in the backscatter direction, suspended at midwater, and insonified by a broadband directional transducer. From the inspection of the response of the solid‐filled shell, it was possible to detect and characterize inhomogeneities of the interior (air inter‐layers), the presence of which were later confirmed by CT scan and ultrasound measurements. Elastic wave analysis and modeling tools supported the physical interpretation of the measured responses. A spherical shell partially filled with epoxy resin and water was found to exhibit a strong focusing effect in the backscattering direction, for ne...